The present invention claims priority to Japanese Patent Application No. 2001-086930 filed on Mar. 26, 2001.
1. Field of the Invention
The present invention relates to processes for fabricating semiconductor devices with a high dielectric constant capacitor.
2. Description of the Background
Semiconductor devices such as dynamic random access memories (DRAM) have achieved high capacity through reduction in the cell area. An inevitable consequence of this reduction is a decrease in the area available for capacitors. Nevertheless, it is still essential for a semiconductor device to store a certain amount of stored charge necessary for memory reading without soft errors. In other words, for high capacity semiconductor devices, it is necessary to provide some means by which to increase the amount of stored charge per unit area. One way to address this need is to form the capacitor insulating film from an oxide dielectric material having a high dielectric constant.
A conventional capacitor insulating film used for memory LSI (Large Scale Integration) is SiO2 film (with a dielectric constant of 3.8) or Si3N4 film (with a dielectric constant of 7 to 8). There materials are currently being replaced by Ta2O5 film (with a dielectric constant higher than 20). To store a much larger amount of charge, it has been proposed to form the capacitor insulating film from an oxide dielectric material having a dielectric constant greater than 100, as exemplified by strontium titanate (SrTiO3, xe2x80x9cSTOxe2x80x9d for short), barium strontium titanate ((Ba,Sr)TiO3, xe2x80x9cBSTxe2x80x9d for short), lead titanate zirconate (Pb(Zr,Ti)3, xe2x80x9cPZTxe2x80x9d for short), or bismuth-based laminar ferroelectrics. However, these oxide dielectric materials (including Ta2O5) have the disadvantage of requiring post heat treatment at high temperatures (e.g., 300-700xc2x0 C.) in an oxidizing atmosphere to obtain these improved electrical properties. This heat treatment typically causes the lower electrode to be oxidized by oxygen in the oxidizing atmosphere, which may result in an insulating film having a lower dielectric constant than the capacitor insulating film. Consequently, there may be a substantial decrease in the capacity of capacitor.
A promising way to address this problem is to form the lower electrode from platinum (Pt), ruthenium (Ru), iridium (Ir), or the like. Pt is comparatively stable when subjected to a high temperature and oxidizing atmosphere. Ru and Ir retain electrical conductivity even after oxide formation. Of these materials, Ru is suitable for microfabrication and is the most desirable as the lower electrode for oxide dielectric material. In addition, a Ru electrode has a large work function and prevents leakage current due to the height of the Schottky barrier at the interface between the capacitor insulating film and the electrode. Therefore, Ru is also a promising material for the upper electrode.
Even though capacitors on a memory LSI of Giga-bit scale may be formed from the Ru electrode and the above-mentioned oxide dielectric material, it is not conventionally possible to secure the amount of stored charge necessary for reading because the area for capacitors is too small. Thus, there arises a need to make the capacitor three-dimensional in order to substantially increase the capacitor area. One way to address this need requires the several steps of: (1) forming a three-dimensional lower electrode from Ru; (2) forming an oxide dielectric material as a capacitor insulating film; and (3) forming an upper electrode from Ru on the oxide dielectric material. These steps essentially involve chemical vapor deposition to form the Ru electrodes.
An exemplary process for forming a Ru film by chemical vapor deposition from an organoruthenium compound as a precursor is described in Japanese Journal of Applied Physics, 38 (1999), p. 2194. This process uses bis(cyclopentadienyl) ruthenium (Ru(C5H5)2) as a raw material to form the Ru film.
The three-dimensional lower and upper electrodes are preferably formed by making deep holes in the surface of silicon oxide film (which permits easy microfabrication) and subsequently depositing Ru by chemical vapor deposition. This procedure may involve the problems explained below with reference to FIG. 11 (a sectional view).
It is assumed that a plug 1 of titanium nitride and an insulating interlayer 2 of SiO2 (in the plug area) have been formed. The procedure starts with depositing a 700-nm thick insulating interlayer 3 of SiO2 (in the capacitor area). Subsequently, deep holes are formed in the insulating interlayer 3 by well-known photolithography and dry etching techniques such that the bottom of the holes reaches the surface of the insulating interlayer 2. These holes preferably have a round, elliptical, or rectangular opening. The diameter of the opening is smaller than 130 nm in a memory LSI of Giga-bit scale.
If each capacitor is to have a capacity larger than 30 fF per bit, the insulating interlayer 3 should have a thickness (equivalent to a hole depth) of at least 700 nm, even in the case where the insulating film of BST has a thickness of 0.4 nm in terms of SiO2. Thus, the deep hole requires an aspect ratio no smaller than 5, wherein the xe2x80x9caspect ratioxe2x80x9d is defined as the ratio of a hole depth to an opening diameter. If the capacitor insulating film is formed from Ta2O5 with a thickness of 0.8 nm in terms of SiO2, then the insulating interlayer 3 needs to be thicker than 1500 nm or the hole needs to be deeper than 1500 nm (with an aspect ratio of 11 at least). In this deep hole is formed the lower ruthenium electrode 10 (20 nm thick) by chemical vapor deposition. See, FIG. 11(a).
Several potential problems as pointed out below are involved in the conventional chemical vapor deposition from an organoruthenium compound as a precursor. According to the above-mentioned paper, the procedure uses bis(cyclopentadienyl) ruthenium (C5H5)2) as a precursor, and the ruthenium film is formed by reactions in two stages. The reaction in the first stage is limited by the surface reaction which has an activation energy of 2.48 eV at a film-forming temperature no higher than 250xc2x0 C. and at an oxygen partial pressure of 0.07 Torr. As the temperature exceeds 250xc2x0 C., the reaction in the second stage takes place which is limited by the mass transport. In this second stage, the film forming rate becomes constant at approximately 23 nm/min. This procedure results in such a high conformality so as to form a ruthenium film with a step coverage of nearly 100% in a deep hole (having a diameter of 130 nm and an aspect ratio of 4) at a film-forming temperature of 230xc2x0 C. Unfortunately, the film-forming rate at 230xc2x0 C. is only about 2.8 nm/min. Therefore, because it takes about 7 minutes to form a 20-nm thick film, the procedure mentioned above may not be suitable for mass production because of the low throughput. In addition, if the above procedure is to be applied to the capacitors of a Giga-bit DRAM, it should be able to form ruthenium film for the upper electrode in a deep hole having an aspect ratio no smaller than 12, even in a case where BST is used for the capacitor insulating film.
After the ruthenium film has been formed as shown in FIG. 11(a), subsequent steps are carried out in the following manner. The lower electrode which has been deposited on the insulating interlayer 3 is removed by sputter etching so as to electrically isolate adjacent capacitors from each other. Thus, a three-dimensional lower electrode structure is obtained, as shown in FIG. 11(b).
Subsequently, an oxide dielectric material 6 is the deposited by chemical vapor deposition. In the case of BST, the thickness of the deposit is 20 nm. In the case of Ta2O5, the thickness of deposit is 10 nm. Thereafter, the upper ruthenium electrode 7 is formed by chemical vapor deposition. In this way the capacitor is completed as shown in FIG. 11(c). It is necessary that the upper ruthenium electrode should completely cover the inside wall of the deep hole having a diameter no larger than 60 nm, a depth no smaller than 700 nm, and an aspect ratio no smaller than 12. It should be noted that the xe2x80x9cstep coveragexe2x80x9d is defined as a ratio of b/a, where a is the thickness of the film deposited on the surface surrounding a hole and b is the thickness of the film deposited on the sidewall near the lower part of a hole (or b may represent the thickness of the film at the thinnest part).
The foregoing may be summarized as follows. The chemical vapor deposition of an organoruthenium compound should be carried out under surface reaction conditions that permits deposition on the sidewall of a deep hole having an aspect ratio of at least 12. In addition, it is also necessary that the film-forming rate should be high (or the reaction probability should be high) for high productivity, and the reaction should take place without an incubation period.
In at least one embodiment, the present invention preferably provides a process for fabricating semiconductor devices involving the efficient formation of ruthenium film with good conformality (i.e., an even thickness or a step coverage of approximately 1). An exemplary process according to the present invention to achieve the above comprises a step of forming a ruthenium film on a surface of a substrate having holes by chemical vapor deposition from an organoruthenium compound as a precursor. This step is preferably carried out in the presence of oxygen and a gas which prevents the substrate surface from adsorbing oxygen in such a way that the decomposition of the organoruthenium compound on the surface is adequately controlled.
In addition, the process of the present invention comprises a-step of forming the lower and upper electrodes of a high dielectric constant capacitor for the semiconductor device by chemical vapor deposition from an organoruthenium compound as a precursor. This step is carried out in the presence of oxygen and a gas which prevents the substrate surface from adsorbing oxygen in such a way that the decomposition of the organoruthenium compound on the surface is adequately controlled.
In either case, the gas used to prevent oxygen adsorption should preferably be one which is liquid at room temperature, has a boiling point no higher than 150xc2x0 C., and dissolves the organoruthenium compound in a concentration of at least 0.001 mol/L. The gas should preferably be at least one compound selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, isobutyl alcohol, 1-butanol, 2-butanol, diethyl ether, diisopropyl ether, tetrahydrofuran, tetrahydropyran, 1,4-dioxane, acetone, methyl ethyl ketone, and toluene.
Additionally, the chemical vapor deposition should preferably be preceded by a step of forming a seed layer comprised of at least one metal selected from the group consisting of ruthenium, platinum, iridium, rhodium, osmium, palladium, cobalt, iron, and alloys thereof.
The above-mentioned method makes it possible to control the amount of oxygen adsorbed on the surface inside the hole. The process of the present invention is explained more fully below in which the organoruthenium compound is bis(ethylcyclopentadienyl)ruthenium (Ru(C2H5C5H4)2 or Ru(EtCp)2) and the oxygen absorption preventing gas is tetrahydrofuran (THF).
Ru(EtCp)2 is used in the form of gas after heating and vaporization (liquid bubbling method). THF is introduced into the reaction chamber in the form of a gas after heating (at least 80xc2x0 C.) for vaporization. The partial pressure of each gas in the mixture (which is comprised of the organoruthenium compound, oxidizing gas, inert gas, and oxygen adsorption preventing gas which are present in the film-forming chamber) is defined as the value obtained by multiplying the mole fraction of each gas supplied by the total pressure of the mixture gas.
The film-forming rate depends on the film-forming temperature as shown in FIG. 6. Film forming was carried out on SiO2 or Ru in the absence or presence of THF (at a partial pressure of 2.8 Torr), at an oxygen partial pressure of 0.24 Torr. A Ru layer was previously formed in the hole by the long throw sputtering (LTS method). When a 50-nm thick film was formed on the flat surface at a film-forming temperature of 300xc2x0 C., a Ru seed layer (about 1-2 nm thick) was formed on the inside wall of the hole. It should be noted that the hole has a diameter of 130 nm, a depth of 800 nm, and an aspect ratio of 6.1.
In the case where THF is not introduced, the film-forming rate is limited by the surface reaction with an activation energy of 0.37 eV regardless of the underlayer at a film-forming temperature below 300xc2x0 C. This surface reaction is limited by the amount of oxygen adsorbed onto the surface, as explained below with reference to FIG. 7. At a film-forming temperature above 300xc2x0 C., the reaction is limited by the amount of precursor supplied. The conformality is comparatively good in the reactions which are limited by the amount of oxygen adsorbed onto the surface; however, the step coverage is only about 40% at a film-forming temperature of 230xc2x0 C. This reaction has a certain amount of incubation period before the formation of the Ru film commences. However, the incubation period for the film on Ru is shorter than that for the film on SiO2. At a film-forming temperature of 230xc2x0 C., there is an incubation period of 5 minutes for the film on SiO2 but there is no incubation period for the film on Ru. This may occur because more oxygen is adsorbed on Ru in a shorter time.
By contrast, the film-forming reaction differs greatly depending on whether the film is formed on SiO2 or Ru in the presence of THF at a partial pressure of 2.8 Torr. On SiO2, the Ru film is formed by an oxidative decomposition reaction with an activation energy of 0.37 eV. However, the reaction region is shifted higher by about 130xc2x0 C. compared to the case where no THf is introduced. This suggests that THF prevents oxygen adsorption in the low temperature region. An incubation period exists as in the case where no THF gas is introduced, and the step coverage is less than 20%. By contrast, the Ru film on Ru, unlike on SiO2, is formed at nearly a constant rate at film-forming temperature of at least 200xc2x0 C. No incubation period exists, and the step coverage in the hole is almost 100%.
In order to elucidate the above-mentioned reactions, experiments were carried out in which the partial pressure of THF was kept constant at 2.8 Torr and the partial pressure of oxygen was varied. The step coverage and the film-forming rate depend on the oxygen partial pressure as shown in FIG. 7(a) and FIG. 7(b), respectively. The film-forming temperature is 300xc2x0 C. and the film-forming time is 2 minutes. It is noted from FIGS. 7(a) and 7(b) that reactions take place differently depending on whether the oxygen partial pressure is lower than or higher than about 0.4 Torr. It is also noted that the rate of film forming on Ru increases linearly in the region of low oxygen partial pressure (at 0.07 Torr and above) and reaches a plateau in the region of high oxygen partial pressure. By contrast, the step coverage uniformly (but not necessarily linearly) decreases in the region of low oxygen partial pressure and converges to a constant value of 50% in the region of high oxygen partial pressure. FIG. 7(b) also shows the rate of film forming on SiO2. It is noted that the film-forming rate increases steeply in the region of high oxygen partial pressure at around 0.4 Torr and above. The film-forming rate in the region of low oxygen partial pressure below about 0.4 Torr is zero because of an incubation period longer than 2 minutes.
The mechanism of film forming on Ru differs depending on the oxygen partial pressure. In the region of low oxygen partial pressure below approximately 0.4 Torr, the reaction is limited by the oxygen supply because the sticking probability increases in proportion to oxygen partial pressure. Conformality in this region is good because the surface reaction depends on the amount of oxygen adsorbed, and the organoruthenium compound and oxygen diffuse into the inside of the hole so that the density of oxygen adsorption is equal to that on the substrate surface. On the other hand, the rate of film forming in the region of high oxygen partial pressure does not vary regardless of the oxygen partial pressure, and the reaction is limited by the supply of precursor in the temperature region for the saturated film-forming rate as shown in FIG. 7(a). It is considered that conformality is poor because the precursor gas is captured by the flat surface of the substrate, and the supply of precursor gas into the inside of the hole is limited.
The effect of the partial pressure of THF gas on the step coverage is evaluated in FIG. 8. FIG. 8 shows how the step coverage and film-forming rate depend on the partial pressure of THF gas, with the partial pressure of oxygen kept constant at 0.11 Torr. Experiments were performed on three kinds of substrates varying in the aspect ratio of holes. On the inside of the hole, a Ru seed layer (1-2 nm thick) was formed by long throw sputtering. The hole with the largest aspect ratio (15.4) has a diameter of 130 nm and a depth of 2000 nm. When the partial pressure of THF is lower than 1.0 Torr, the step coverage and film-forming rate do not depend on the partial pressure of THF gas. There is no difference from those in the conventional case where THF (as the gas to prevent oxygen adsorption) is not present. However, at a partial pressure of THF gas in excess of 1.0 Torr, the step coverage increases. Further, when the partial pressure of THF gas reaches 2.9 Torr, the step coverage is nearly 100% (at an aspect ratio of 15.4). In the case of a hole with a small aspect ratio, the partial pressure of THF gas that permits 100% step coverage is as low as 1.5 Torr. On the other hand, the film-forming rate decreases as the partial pressure of THF gas increases.
The foregoing suggests that the partial pressure of THF gas controls the oxygen adsorption density and the distribution of oxygen density in the hole. It is considered that when the partial pressure of THF gas is small, the oxygen supplied is mostly adsorbed by the patterned flat part or the upper inside of the hole and hence does not reach deep inside the hole. Thus, it is further considered that the oxidation decomposition reaction of the organoruthenium compound, which is limited by the amount of oxygen adsorbed, proceeds, thereby aggravating conformality at the bottom of the hole. On the other hand, it is considered that when the partial pressure of THF is high, THF gas sufficiently adsorbs to the surface of the Ru seed layer, thereby preventing the adsorption of oxygen but permitting the diffusion of oxygen to the bottom of the hole, and a constant low density of oxygen adsorption is realized. This causes good conformality at the bottom of the hole. In addition, it is considered that the density of oxygen adsorption decreases with an increasing partial pressure of THF gas, and, hence, the film-forming rate also decreases as shown in FIG. 8(b).
FIGS. 6 through 8 show that the introduction of THF gas controls the state of oxygen adsorption on the substrate surface and permits the Ru film to be formed with good conformality at a high rate without an incubation period. Since the Ru film is formed by the decomposition reaction of a precursor which is limited by the density of oxygen adsorbed, it is possible to optimize the supply of precursor. The present invention addresses the limitations of the conventional technology by preferably providing a process in which the film-forming time is shorter and the precursor is used more efficiently, which leads to increased productivity and decreased production cost. Thus the present invention may be useful for forming capacitor electrodes in Giga-scale DRAMs.
The above-mentioned effect may also be obtained even though THF (as the oxygen adsorption preventing gas) is replaced by at least one compound selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, isobutyl alcohol, 1-butanol, 2-butanol, diethyl ether, diisopropyl ether, tetrahydropyran, 1,4-dioxane, acetone, methyl ethyl ketone, and toluene.
It is possible to simultaneously supply both the organoruthenium compound and the oxygen adsorption preventing gas if the former is dissolved in the latter in liquid form (e.g., THF and the like mentioned above) and the resulting diluted solution is vaporized by heating. It is also possible to adequately control the partial pressure of oxygen and the partial pressure of gas (evolving from the diluted solution) in the film-forming chamber if the supply of the diluted solution is controlled and the oxygen adsorption preventing gas is introduced through another route. In practice, for example, Ru(EtCp)2 as an organoruthenium compound may be dissolved in THF, and the resulting solution is fed after vaporization in a constant temperature bath at 150xc2x0 C. If the partial pressure of oxygen and THF gas is adjusted to 0.1 Torr and 3.0 Torr in the film-forming chamber, respectively, it is possible to realize the reaction condition which permits high step coverage as mentioned above.
The organoruthenium compounds that may be used, in addition to the bis(ethylcyclopentadienyl)ruthenium (Ru(C2H5C5H4)2) mentioned above include: bis(cyclopentadienyl)ruthenium (Ru(C2H5)2); bis(methylcyclopentadienyl)ruthenium (Ru(CH3C5H4)2); and tris (dipivaloylmethanate) ruthenium (Ru(C11H19O2)3): These compounds undergo chemical vapor deposition by the same mechanism as that of bis(ethylcyclopentadienyl)ruthenium (Ru(C2H5C5H4)2). These organoruthenium compounds permit the substrate temperature and oxygen partial pressure to be optimally controlled if the oxygen adsorption preventing gas is introduced.
Table 1 shows the substrate temperature, the partial pressure of oxygen, and the partial pressure of oxygen adsorption preventing gas which are optimal for the individual organoruthenium compounds. Any organoruthenium compound other than those listed in Table 1 may be used for the above-mentioned film-forming process so long as it is an organic compound of noble metal which is decomposed by surface-adsorbed oxygen.
A ruthenium film with good conformality is formed by supplying a gas which is hardly adsorbed to Ru surface in the following manner. In order to deposit a ruthenium film with good conformality as mentioned above, it is necessary to keep the adsorption density of oxidizing gas on the growth surface (Ru surface) constant and low. An example of a practical procedure is given below in the case where the organoruthenium compound is Ru(C5H4C2H5)2 or Ru(EtCp)2 and the oxidizing gas is oxygen. The precursor is supplied by the liquid bubbling method.
The procedure starts with the formation of a Ru seed layer (20 nm thick) by sputtering, and then the desired film is formed. The seed layer reduces the incubation period required for growth nucleation. The supply of oxygen and the supply of argon (not as a carrier gas) are controlled so that oxygen and argon each have a desired partial pressure.
FIG. 13 shows how the activation energy for decomposition reaction of Ru(EtCp)2 varies with the partial pressure of O2 and the partial pressure of Ar. It is noted from FIG. 13 that a shift takes place from the decomposition mechanism with an activation energy of about 1.4 eV to the decomposition mechanism with an activation energy of about 0.4 eV at a certain partial pressure of oxygen, which rarely depends on the partial pressure of argon. It is also noted that the lower the partial pressure of oxygen, the better the conformality, with very little dependence of the partial pressure of argon. Hatching in FIG. 13 denotes the region for high step coverage (with an aspect ratio of 15). It is noted that good coverage of Ru film is formed on the sidewall of a deep hole with an aspect ratio of 15 when the partial pressure of oxygen is lower than 0.007 Torr, regardless of the partial pressure of argon. This suggests that the density of oxygen adsorbed on the surface is determined mainly by the partial pressure of oxygen.
The same results as above were obtained with a rare gas (such as Ne, He, and Xe) in place of argon. In the case of nitrogen (N2) and carbon dioxide (CO2), the partial pressure of oxygen (which changes the activation energy) shifts to the higher side as their partial pressure increases, as shown in FIG. 13. In this case, the high-coverage region (hatched with horizontal lines in FIG. 13) shifts to the higher partial pressure of oxygen by no more than about 0.01 Torr. However, this change is smaller as compared with that shown in FIG. 14 (explained below). It is therefore considered that the density of oxygen adsorbed to the surface is determined almost entirely by the partial pressure of oxygen in this case too.
These gases (Ar, He, Xe, N2, and CO2) have the common disadvantage in that they are not adsorbed to Ru as easily as oxygen. This implies that the absolute value of their heat of adsorption on Ru is smaller than that of oxygen. It is reasonable to consider that the density of adsorption of oxygen on the surface of Ru is determined mainly by the partial pressure of oxygen. One effective way to improve conformality is to increase the supply of a gas hardly adsorbable to Ru, thereby decreasing the partial pressure of oxygen. This is accomplished by controlling the amount of exhaust such that the total pressure in the film-forming chamber is kept constant (at 5 Torr, for instance).
For good conformality in practice, it is necessary to control the partial pressure of oxygen in the film-forming chamber so that it is as low as 0.01 Torr, as shown in FIG. 13. It is also necessary to adjust the position of the shower head and the amount of gas supply so that the partial pressure of oxygen does not fluctuate on the wafer. Despite such adjustment, it may be difficult to keep the partial pressure of oxygen constant when the partial pressure of oxygen is lower than 0.003 Torr, because under such conditions, the thickness of the Ru film varies greatly on the wafer.
In the above-mentioned experiments, Ru(C5H4C2H5)2 is used as a typical precursor for CVD. The same effect may also be produced even if it is replaced by any of Ru(OD)3, Ru(CH3C5H4)2, Ru(C5H5)2, and Ru(C11H19O2)3. (The latter three are solid at room temperature.) The gases to be introduced to reduce the partial pressure of oxygen include Ar, Ne, He, Xe, N2, and CO2, which are hardly adsorbed to Ru. The same effect is also produced by a non-polar hydrocarbon compound without multiple bonds. In the above-mentioned experiments, oxygen is used to decompose the precursor for CVD; the same effect is also produced if the oxygen is replaced by an oxidizing gas such as N2O, H2O, NO, and O3. The optimal substrate temperature is approximately 200-400xc2x0 C.
A ruthenium film with good conformality may also be formed by supplying a gas which is readily adsorbed to Ru surface in the following manner. In this case, the organo-ruthenium compound as a precursor is Ru(EtCp)2, the oxidizing gas is oxygen, and the gas readily absorbable to Ru surface is THF. The precursor is supplied by the liquid bubbling method employing argon as a carrier gas. The procedure begins with the formation of a Ru seed layer (20 nm thick) by sputtering, and then the desired film is formed. The seed layer reduces the incubation period required for growth nucleation. The supply of oxygen, the supply of THF, and the supply of argon (not as a carrier gas) are controlled so that oxygen and THF each have a desired partial pressure. The pressure in the film-forming chamber is kept at 5 Torr by controlling the amount of exhaust.
FIG. 14 shows how the activation energy for decomposition reaction of Ru(EtCp)2 varies with the partial pressure of oxygen and the partial pressure of THF. It is noted from FIG. 14 that a shift takes place from the decomposition mechanism with an activation energy of about 1.4 eV to the decomposition mechanism with an activation energy of about 0.4 eV at a certain partial pressure of oxygen, which increases with the increasing partial pressure of THF. It is also noted that the lower the partial pressure of oxygen and the higher the partial pressure of THF, the better the conformality. Hatching in FIG. 14 denotes the region for high step coverage (with an aspect ratio of 15).
It is noted that Ru film with good coverage is formed in a deep hole with an aspect ratio of 15 when the supply of THF liquid is 5 sccm, the supply of oxygen gas is 50 sccm, and the supply of total argon (including carrier gas) is 900 sccm. Under these conditions, the partial pressure of oxygen and THF in the film-forming chamber are 0.11 Torr and 2.95 Torr, respectively. This implies that the density of oxygen adsorbed on the surface is determined by both the partial pressure of oxygen and the partial pressure of THF. The density of oxygen adsorbed on the surface can be reduced by increasing the partial pressure of THF. The same result may be obtained when THF is replaced by carbon monoxide (CO), ethylene (C2H4), or acetylene (C2H2). As shown in FIG. 14, it is possible to reduce the density of oxygen adsorbed on the Ru surface by increasing their partial pressure.
The results shown in FIG. 14 may be better understood by examining the adsorption of mixed gas on Ru surface. Gases such as THF, CO, C2H4, and C2H2 are characterized by a larger absolute value of heat of adsorption on Ru surface than the above-mentioned inert gases such as Ar, Ne, Xe, N2, and CO2. This implies that they are readily adsorbed to the Ru surface. In general, when a mixed gas is adsorbed to a metal surface, the density of individual gases adsorbed is determined by the partial pressure of each gas, heat of adsorption, density of saturated adsorption, substrate temperature, and surface roughness. In other words, if a gas, such as THF which is readily adsorbed to Ru surface, is introduced and its partial pressure is controlled, it is possible to change the ratio of the density of an oxidizing gas adsorbed to Ru surface to the density of other gases adsorbed to Ru surface.
Reducing the density of oxygen adsorbed to the Ru surface by introducing a gas which is readily adsorbed to the Ru surface is an effective way to improve conformality. The introduction of these gases makes it possible to set the optimal oxygen partial pressure above 0.01 Torr, and this offers the advantage of keeping the distribution of oxygen partial pressure uniform on the wafer and to minimize the variation of Ru film thickness and step coverage on the water surface.
In the above-mentioned experiments, Ru(C5H4C2H5)2 was used as a typical precursor for CVD. The same effect is also produced if it is replaced by any of Ru(OD)3, Ru(CH3C5H4)2, Ru(C5H5)2, and Ru(C11H19O2)3. (The latter three are solid at room temperature.) The gases to be introduced to reduce the density of oxygen adsorbed on Ru include THF, CO, C2H4, and C2H2 (with polar, multiple bond structure) which are readily adsorbed to Ru. Those which are liquid at room temperature can also be used if they have a boiling point lower than 150xc2x0 C. and can be vaporized for supply.
The condition for high conformality is indicated by the hatched region in FIG. 14. Also, since the above-mentioned organoruthenium compound itself, such as Ru(C5H4C2H5)2, is readily adsorbed to the Ru surface, its increased supply decreases the amount of oxygen adsorbed and increases the step coverage. For improved conformality, the amount of organoruthenium compound should be more than about one-fifth of the amount of oxygen in terms of gas. This leads to an increased consumption of Ru precursor and an increased production cost. In the above-mentioned experiments, oxygen is used to decompose the precursor for CVD; the same effect is also be produced if the oxygen is replaced by an oxidizing gas such as N2O, H2O, NO, and O3. The optimal substrate temperature is approximately 200-400xc2x0 C. irrespective of the kind of organoruthenium compound and oxidizing gas and other gas used.